pylorus along the circular fibres, so as to look like a broad ring of constriction progressing slowly downward.

The longitudinal fibres at the same time contract so as to shorten the piece of intestine immediately below the ring of constriction, and also cause a certain amount of rolling movement of those loops of intestine which are free enough to move. This motion takes place periodically in proportion to the amount and character of the contents of the intestine, the food passing over the mucous membrane being to all appearance the stimulus which normally calls forth and intensifies the action.

FIG. 55.

a

с

jd

e

The activity of the peristaltic movements varies with many circumstances besides the con

f tents of the intestines. Of these

the most noticeable is the amount

Diagram of a longitudinal section of the and character of the blood flow

a. Villi.

b. Lieberkühn's Glands.

c. Tunica muscularis mucosa, below which lies Meissner's nerve plexus.

d. Connective tissue in which blood and lymph vessels lie.

many

e. Circular muscle fibres cut across with Auerbach's nerve plexus below it.

f. Longitudinal muscle fibres. g. Serous coat.

ing through the vessels of the intestinal wall. Thus stoppage of the blood current by tying

the arteries, or deficiency of oxygen and excess of carbonic acid, causes inordinate activity of the peristaltic action. Direct irrita

tion of the serous surface of the intestine with mechanical, chemical, or electrical stimuli also causes an increase in the movements of the intestine.

The great activity of the motion observed when the abdominal cavity of a recently killed animal is opened depends partly on the exposure to cool air, and partly on the venous character of the blood in the vessels no longer oxidized by respiration.

The irregular and impetuous action of the intestine which follows the constriction or strangulation of a hernial protrusion, probably depends chiefly on the mechanical stimulation, but also

is intimately related to interference with the blood supply consequent on the pressure exerted by the constricting band. Prolonged overwork often induces immobility of the intestinal wall, and hence we find the purging and vomiting which accompany a temporary hernial constriction followed by inability of the intestine to propel its contents. These points have also been proved by results of experiments on the lower animals.

The movements of the large intestines are the same as the small, but not so obvious, owing to the modified sacculated shape of this part of the alimentary canal. The contractions of the colon begin at the ileo-cæcal valve where the peristaltic wave of the ileum ceases. The normal intestinal motions thus pass in an almost uninterrupted wave from the pylorus to the end of the gut, but when special sources of irritation exist, a wave may originate in almost any intermediate part of the intestine. A reversed" anti-peristaltic motion," as it is called, only occurs as a result of some intense local stimulation, such as the strangulation of a hernia, etc.

The motion produced by the substances contained in the intestine depends on their character. The solid parts excite more rapid movements, and the more fluid portions but slightly influence the intestinal peristalsis.

Thus the solids which make their way through the pylorus are seldom to be found in the jejunum, no matter at what period after a meal the animal be killed, whereas the folds of the mucous membrane are always bathed in a fluid, creamy material during the entire period of digestion, and even for a considerable time after all the food has left the stomach.

Mechanism of Defecation.-This is a point of much importance, for the evacuation of the lower bowel is intimately connected with feelings of comfort and health, and in illness the insuring of its accomplishment forms an essential part of the physician's duty.

The movements of the intestine cause the various excretions and indigestible parts of the food to pass toward the sigmoid flexure of the colon, where their onward motion is checked for a

time by the strong circular muscle of the rectum (called the superior, or tertius sphincter by Hyrtl), which does not carry on the peristaltic wave. The materials here get packed into a more or less solid mass, which is gradually augmented after each meal.

The lower outlet of the alimentary canal is closed by two distinct sphincter muscles. One thin external superficial muscle, made up of striated fibres, belongs to the perineal group, and has little influence on the closure of the anus. The deep or internal sphincter, which is much stronger, surrounds the gut for rather more than an inch (3 centimetres, Henle) in height, and is onequarter inch thick. It is made of smooth muscle, and therefore capable of prolonged (tonic) contraction. It would appear, however, that this strong sphincter is merely a supernumerary guard to the anal orifice, but rarely called into action, for during the interval of rest between the acts of defecation, the fæces do not come in contact with the portion of intestine surrounded by this muscle. The rectum for quite one inch above the sphincter is perfectly empty, being kept free from feculent particles partly by a fold of the intestinal wall and partly by the repeated action of the voluntary muscles in the neighborhood, which, by intensifying the angle that exists at this point and flattening this inch of rectum, can squeeze back the approaching matters. Any one familiar with the digital examination of the unevacuated rectum, knows that no fæces are met with for about two inches.

Considerable accumulation may take place in the sigmoid flexure without much discomfort ensuing, but when the rectum is distended, an urgent sensation of wanting to empty it is experienced, and the voluntary movements mentioned above are performed by the levator ani and the neighboring perineal muscles, with the object of preventing any substance reaching the part of the rectum immediately above the sphincter.

If the rectum be distended with fluid, the occasional anal elevation does not suffice to keep it back, and a continuous and combined action of the sphincters and levator ani, etc., is necessary to ward off the expulsion of the contents.

When the lower bowel is habitually emptied at the same hour

daily-a habit which should be carefully exercised-the sensations of requirement to go to stool occur with great punctuality, or can be readily induced by the will, so that normal defecation is reputed to be, and practically is, a voluntary act. But not completely so, for, somewhat like swallowing, the later stages of defecation consist essentially of a series of involuntary reflex events which we can initiate by the will, but when it is once started, are powerless to modify until the reflex sequence is completed.

Under ordinary circumstances, the evacuation of the fæces is commenced by the voluntary pressure exercised on the abdominal contents by the respiratory muscles. The diaphragm is depressed,

the outlet of the air passages firmly closed, and the expiratory muscles thrown into action, while at the same moment the muscles which close the pelvic outlet relax, and allow the anus to descend, so that the inferior angle of the rectum is straightened, and a voluntary inhibition of the sphincter is brought about. This voluntary expiratory effort seldom requires to be continued for more than three or four seconds before some fecal matter reaches the part of the rectum just above the sphincter. When this has occurred, no further abdominal pressure is necessary (except when the masses of fæces are large and hard), for the local stimulus starts a series of reflex acts which carry on the operation.

These consist of an increased peristaltic contraction of the colon and sigmoid flexure, the waves of which pass along the rectum. These waves are accompanied by synchronous rhythmical relaxation of the sphincter, which replaces its normal condition of tonic contraction.

The effect of the voluntary effort, and the amount of the abdominal pressure required, depend upon the consistence of the fæces. When quite fluid, they constantly tend to come in contact with the sensitive point of the rectum, and a voluntary effort is required to prevent the reflex series of events from taking place ; a momentary relaxation of the sphincter with voluntary abdominal pressure is sufficient to eject the contents of the bowel. On the other hand, when the fæces are firm, time is required in order that the slowly acting smooth muscle may pass the mass onward. In common constipation, the difficulty is to get the solid mass down to the sensitive exciting point, in which case a few drachms of warm fluid, used as an enema, may awaken the necessary reflex movements.

Nervous Mechanism of the Intestinal Motion.-Many points in the nervous control exerted over the intestinal muscles are obscure. We know that intestinal movements which are peristaltic in their nature occur in a portion of intestine removed from the body, and thus separated from all central nervous control. We know, also, that there are abundant nerve elements in the walls of the intestines which have all the characters of ganglion cells, and therefore probably act as nerve centres. (Figs. 56, 57.)

With regard to these local nervous agencies, anatomists have made out two distinct sets, both of which have the form of a network of nerve fibrils studded with cell elements at their nodal points. One of these, a closely-meshed plexus with flattened cords and ganglionic masses at their points of union, lies between the longitudinal and circular layers of muscle (Figs. 56, 57), forming the plexus myentericus exterior of Auerbach, and most probably controls the movements of these layers of muscle. The other lies internal to the circular muscle, in close relation to the